As the bloke on the Irish roadside says I wouldn't start from here: ) Milling on the lathe is always a bodge. The vertical slide is clumsy, the capacity is rubbish, the travel nowhere near enough, the overhangs and resultant rigidity rubbish, the visibility of the cutting edge often non existent. Holding cutters needs an effective collet chuck and they are quite expensive. Horizontal boring is a little easier but still a huge compromise.
For the money the original attachments cost you could probably pick up an import milling machine and do the job properly.
My Boxford had essentially just the chucks and a single 'american' toolpost when acquired, now some 40 years later It's now relatively fully tooled with taper attachment, 3C collets, ER collets, vertical slide, slotted crosslide and a boring table. The 4 way toolpost foolishly bought in the late 1980's now replaced by a model 100 interchangeable one from arceurotrade and insert tooling more or less entirely replacing HSS and brazed carbide.
Without workshop space for a milling machine I never bought one until about a decade ago and so milling on the lathe became a necessity.
But, for other than a very small handful of jobs over the years the task of milling is so much easier on a machine actually designed for the task. The vertical slide was iirc last used in the 20th century and the last time I recall using the boring table was about 2010 ish for a one off horizontal boring job that I recall well as it took a couple of weeks to set up as it required extensive jig and clamp manufacture and just half a saturday morning to machine a couple of mm from a 30mm bore for 70mm length in aluminium.
I'll never part with the boxford attachments because a job might come up that actually needs them, but for most of my milling work I would never even contemplate using the lathe as even a cheapy import milling machine like my Sieg X3 does a far better job.